Thermoplastic polymers such as e.g. polyvinyl chloride ("PVC") almost always contain processing agents, which impart improved physical properties to the thermoplastic. Depending on the composition of the thermoplastic, processing of the thermoplastic may not be possible without the processing agent. Thus, the processing agents bring about better working properties, the melting and flow behavior is improved, the polymer and optionally added additives such as stabilizers, external and internal lubricants, pigments, fillers, adhesion promotors, co-stabilizers, agents for increasing impact resistance, softeners, etc. can be homogenized better and more rapidly.
In particular, softener-free polyvinylchloride (PVC) is frequently processed to moldings such as e.g. plates, foils, pipes, foam parts and formed materials of many shapes. However, PVC in particular is difficult to process without processing agents since the thermoplastic tends to decompose before reaching the molten state. For this reason, processing agents have long been known and used which are intended to improve the processing and sliding properties in PVC and other thermoplastics. Thus, e.g., Published German Patent Application DE-OS 19 18 893 describes processing agents based on polyacrylate and suitable for various thermoplastics. In those agents, a copolymer of longer-chain and shorter-chain alkylacrylates imparts sliding properties to the processing agent. This involves a rubber-like polymer with unfavorable handling properties, the product tends during a concentration process, e.g. spray drying, to adhere and can therefore only be used as a solution.
The same problems are also presented by a processing agent for PVC polymers described in Published German Patent Application DE-OS 21 23 384. The agent described in that document essentially contains acrylic ester with C.sub.4 -C.sub.8 alcohols and up to 9% by weight acrylonitrile. The processing agent described in that publication is prepared by means of the polymerization of monomers in fatty alcohols which are intended to impart to the thermoplastics at a later time the necessary sliding properties for processing on calender rollers.
A similar processing agent is disclosed in British Patent 981,116 which consists of a copolymer of methacrylates with short alcohol and acrylate units with somewhat longer alcohols. In PVC, only the short alcohols (C.sub.1 -C.sub.4) are desired in the acrylate component. Otherwise, the processing agent becomes incompatible with the thermoplastic.
Short-chain processing agents based on polymethyl methacrylate ("PMMA") are known from British Patent 1,347,287. These short-chain polyacrylates function as flow improvers. They are less suitable for PVC processing on calender rollers, since the thermoplastic bakes too rapidly on the calender rollers.
A further processing agent for PVC is described in U.S. Pat. No. 2,868,763 which is intended to be as compatible as possible with the PVC. To this end, a copolymer of primarily vinyl chloride and a lesser proportion of acrylates is produced, and in particular acrylic esters with a chain length of 6-10 carbon atoms in the alcohol are used. The thermoplastic PVC mixtures obtained can only be processed up to temperatures of slightly above 200.degree. C. since the high proportion of PVC in the processing agent cannot impart sufficient thermostability to the thermoplastic mixture.
Because of the problems described above, multistage processing agents are described in the following patents and Published German Patent Applications: DE 16 94 409; DE 21 35 024 and DE 41 25 477; U.S. Pat. No. 4,436,861; Published European Patent Applications EP 0,061,134; EP 0,184,851 and EP 0,367,198. In the simplest instance, (DE 16 94 490, which represents an improvement over British Patent 981,116) a PVC suspension is taken as the starting point, onto which a mixture of alkylmethacrylates and alkylacrylates with differing length of the alkyl groups is polymerized. The processing agents produced in this manner require considerable amounts of an external lubricant which is intended to separate the melt of the thermoplastic to be processed from the walls of the processing machines used. If too little of the external lubricant is used, the melt can no longer be separated e.g. from calender rollers, even after a short time, and decomposition occurs on the roller. However, the external lubricants have the disadvantage that they exude on account of their incompatibility with the thermoplastic polymer and leave deposits on the walls of the processing machines.
Moreover, external lubricants considerably lengthen the gelling time of the thermoplastic, that is, the time between the start of mixing and the formation of a homogeneous melt of the thermoplastic is considerably lengthened.
In order to reduce the necessary amount of external lubricant, a multiphase acrylic compound polymer is suggested in Published German Patent Application DE 21 35 024 C2, U.S. Pat. No. 4,436,861 and in the European Patent Publications cited above, which primarily contains a soft acrylate phase with longer-chain alkyl groups adjacent to an encasing hard phase. According to the just-mentioned U.S. patent, the acrylic (co)polymer is even grafted onto the base polymer (PVC). The hard phase acts presumably as a processing agent, that is, it influences the viscosity and the gelling time as well as optionally also the sheen and other properties of the thermoplastic polymer; on the other hand, the soft phase has presumably the effect of an external lubricant. It has the advantage that this "lubricant" can be compatible, depending on the composition of the processing agent, with the thermoplastic polymer, so that during the processing of the melt the exuding and the associated formation of a coating are reduced. The soft core is isolated to the outside by the hard polymer shell so that the multi-shell processing agents no longer tend to adhere and can be correspondingly spray-dried. These processing agents can therefore be worked more readily into the thermoplastic polymers. A disadvantage of the multi-shell polymers is the fact that in order to obtain an acceptable lubricating action while retaining the properties of the processing agent, the method of production and the composition must often be coordinated very precisely down to the smallest details such as e.g. the chain transfer agents, so that even rather small variants in the composition or the method result in a product with low-grade properties. Basically, a compromise between the dissolving properties of the machine walls and the gelling time is necessary, as also in the general use of external lubricants, since the gelling time is noticeably lengthened in asssociation with the better lubricating action. In the extreme instance, the gelling can even be eliminated.